This study aims to examine language dominance and language switching effects in a series of monolingual and multilingual lexical decisions in which participants have to decide if the presented letter string is a word or not, regardless of language. Thirty participants (12 French-English bilinguals and 18 French-English-Spanish trilinguals) were recruited for two different experiments. In Experiment 1, 12 bilinguals processed two monolingual lists (L1 and L2) followed by a bilingual list (with L1 and L2 words). The results indicate faster answers and better accuracy for L1 words compared to L2 words in both lists, and a general slowdown of processing in the bilingual list, highlighting language switching effects (switch from L2 to L1 faster than from L1 to L2). In Experiment 2, 18 trilinguals performed successively monolingual (L1, L2 and L3), bilingual (L1L2, L1L3 and L2L3) and trilingual (L1L2L3) lists. The results show a slowdown of word processing as a function of number of languages involved, even for the dominant language (L1). Moreover, a language switching effect was found for all multilingual lists, in larger proportions when the list involves the two weakest languages (L2 and L3). Results are discussed in the light of interactive models of bilingual visual word processing.